Syntactic foams are a special class of composite physical foams, wherein certain hollow microspheres are embedded in matrix binders so as to result in uniform and ordered arrangement of former. A wide variety of hollow microspheres are available, including cenospheres, glass microspheres, carbon and polymer microballoons. The matrix material can be selected from almost any metal, polymer or ceramics. One of the most widely used and studied syntactic foams consist of glass microballoons embedded polymeric binders. Unlike many other chemically synthesized polymeric types of foam, these syntactic foams have excellent mechanical and damping properties owing to the presence of embedded glass reinforcements. The applications of these foams are also widely known in the art. These were initially developed as buoyancy aid materials for marine and submarine applications. Thereafter, other characteristics led these materials to aerospace and ground transportation vehicle applications as well. Among the present applications, some of the common examples are buoyancy modules for marine drilling risers, boat hulls, and parts of helicopters and airplanes.
In U.S. Pat. No. 4,508,640, an electromagnetic wave-shielding material is disclosed. The material is comprised of one or more electromagnetic wave-shielding layers and one or more surface layers. The shielding layers are comprised of a thermoplastic resin, an aluminum or aluminum alloy and electrically conductive carbon black. However, it is very difficult to provide flame resistance to compositions containing large amounts of carbon black.
An electromagnetic interference (EMI) shielding composite is disclosed comprising of reinforcing fibers such as conductive fibers. Since conductive fibers and metallized glass fibers do not provide substantial reinforcement to the composite, it is generally necessary to add additional reinforcing fibers to obtain the desired physical properties. Consequently, the resulting composite is a highly dense material having poor moldability.
Another broadband EMI shielding nanocomposite material, comprising a low-melting metal alloy dispersed in a thermoplastic polymer and a process for making it is also known in the art. Weather-resistant electromagnetic radiation shielding materials and structures for electromagnetic interference attenuation for electronic equipment situated in unshielded outdoor housings have also been described.
Yet another shielding composition made with magnetic particles and a binder is disclosed in U.S. Pat. No. 5,938,979 where the magnetic particles have an average diameter less than about 1000 nm and are substantially crystalline
Carbon nanotubes (CNT) one of the most promising exponents provides multifunctional reinforcements to the composite materials. These are essentially long graphene sheets wrapped in different angles (chiral angles) and with different circumferential lengths (chiral vector), depending upon which they are able to offer a wide range of mechanical, electrical and thermal properties.
However, none of the above described inventions have concentrated on exploiting the capabilities of CNT reinforced syntactic foams in the light of EMI shielding requirements and prospective uses as lightweight multifunctional core materials in subsequent sandwich constructions designed for the same purpose.